Interested in your ability to use the 20% coupon twice, I headed over to REI as I had just used both my dividend and coupon on Apr 3.
It seems that the site is retaining the coupon code and the 20% is being deducted automatically without any coupon entry, nor any opportunity to enter a coupon. It is impossible for me to pay the normal non-discounted price! Reading the details of the coupon, there is no mention of single use that I can find.
From this one can only assume that the coupon is valid on as many separate purchases as one cares to make. No idea if this is what REI intended, but this is what they are, in effect, offering.